Positron emission tomography (PET) has been widely used in medicine for diagnosis and other purposes. A subject, such as a patient, may be scanned with a PET scanner to obtain medical images. A PET scanner includes a plurality of detector units. The detector units are used for detecting coincidence events.
Time-of-flight (TOF) information is generally used for PET image reconstruction. For an annihilation event, the time that each of the coincident photons is detected at two detector units (or referred to as arrival time), and the difference is calculated. Since the travel distances of the two photons to their respective detector units may be different from each other, the photon whose travel distance is shorter may arrive at its detector unit first, compared to the other photon. The difference in the arrival time of the coincident photons may help pin down the location of the annihilation event along the line between the two detector units. Accurate TOF may allow the reconstruction of a PET image.
In general, a phantom is used to calibrate and/or verify the accuracy of a PET scanner. A phantom is a model with a known geometry (e.g., shape, size, etc.) and/or a known distribution of radiation activities throughout the body of the phantom. By imaging the phantom, the accuracy of the imaging apparatus for a three-dimensional or two-dimensional image may be assessed and the settings of the PET scanner may be adjusted based on the phantom data. For instance, the time offset of a detector unit may be calibrated. A phantom may be designed to be a solid body, and the phantom may be placed at the center of the FOV of the PET scanner for a calibration scan. Thus, a device to adjust the position of the phantom is needed. Furthermore, these requests make the phantom imaging process complicated and time-consuming. A method to determine the time offset for detector units is described in this application.